Problems with simple Qt Style Sheet

[Qn00b]

Hello

I have a very simple problem with the setStyleSheet() function. I need to apply a simple style to a label.
If I make my form only with code (without using Qt Designer) I can't apply my style. An example:

Form::Form(QWidget *parent) : QWidget(parent) {
    this->myLabel = new QLabel(this); 
    this->myLabel->move(0,0); 
    this->myLabel->resize(300, 100);
    this->setStyleSheet("#myLabel{background-color: rgb(0, 0, 255);}");
}

Form::Form(QWidget *parent) : QWidget(parent) {
    this->myLabel = new QLabel(this); 
    this->myLabel->move(0,0); 
    this->myLabel->resize(300, 100);
    this->setStyleSheet("#myLabel{background-color: rgb(0, 0, 255);}");
}

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It doesn't work, but if I make my form with Qt Designer it works:

Form::Form(QWidget *parent) : QWidget(parent), ui(new Ui::ClientFrame) { 
    ui->setupUi(this); 
    this->setStyleSheet("#label {background-color: rgb(0, 0, 255);}");
}

Form::Form(QWidget *parent) : QWidget(parent), ui(new Ui::ClientFrame) { 
    ui->setupUi(this); 
    this->setStyleSheet("#label {background-color: rgb(0, 0, 255);}");
}

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Someone can help me to fix the first code please??

Thanks!!